Both packages referred to the same repo (`gflags/gflags` at GitHub),
both packages used almost the same version (2.2.1 and 2.2.0). Therefore
I think that it's just fair to drop one of the packages to avoid
duplication and reduce the maintenance overload.
I decided to drop `gflags` originally packaged by @shlevy as it didn't
contain testing support as `google-gflags` does. The package set refers
from `gflags` to `google-gflags`, so both package names remain (to avoid
any confusions), however the `gflags` expression is removed.
Darwin machines come with a case-insensitive filesystem by default. The
gflags package's source contains a file 'BUILD' and the build process
attempts to create a directory called 'build', which fails on
case-insensitive filesystems.
Add a prePatch hook to rename the BUILD file (which is for use with an
unrelated build tool) to something that doesn't conflict with the
'build' directory. This hook allows this derivation to be built on
case-insensitive filesystems.
Add metadata to the derivation because previously it had none.
